Similarly, take the derivative of x at both ends of the equation and get: f'1-f' 2 (dz/dx)+f' 3 (dz/dx-1) = 0, and you can get dz/dz = (f' 1-f' 3.
The sum of them is 1.
The second is calculated by triple integral, and the integrand function is 1. The integral area is the area enclosed by the two. Product z first, from (x 2+y 2)1/2 to 6-x 2-y 2. The rest is to find the projection on the xoy plane. Is a circle with a radius of 2, and then it is solved by polar coordinate transformation. There should be no problem.